Overview

SIGMETs

A SIGMET
advises of weather, other than convective activity, that is potentially
hazardous to all aircraft. SIGMETs are issued (for the lower 48 states and
adjacent coastal waters) for the following weather-impacted reasons:

Severe Icing

Severe or Extreme Turbulence

Dust storms and/or sand storms lowering visibilities to less than
three (3) miles

Volcanic Ash

These SIGMET items are considered to be widespread because
they must be affecting or be forecast to affect an area of at least 3000
square miles at any one time. However, if the total area to be affected
during the forecast period is very large, it could be that only a small
portion of this total area would be affected at any one time. SIGMETs are issued
for 6 hour periods for conditions associated with hurricanes and 4 hours for
all other events. If conditions persist beyond the forecast period, the
SIGMET is
updated and reissued. Convective SIGMETs are issued hourly for thunderstorm-related
aviation hazards.

Convective SIGMETs

Convective SIGMETs are issued in the conterminous U.S. for:

Severe surface weather including:

surface winds greater than or equal to 50 knots

hail at the surface greater than or equal to 3/4 inches in diameter

tornadoes

Embedded thunderstorms

Line of thunderstorms

Thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at
least 3000 square miles

Any convective SIGMET implies severe or greater turbulence, severe
icing, and low level wind shear. A convective SIGMET may be issued for any convective
situation which the forecaster feels is hazardous to all categories of
aircraft. Bulletins are issued hourly at Hour+55. The text of the bulletin
consists of either an observation and a forecast or just a forecast. The
forecast is valid for up to 2 hours.

CWAs

CWAs are advisories
issued by the Center Weather Service Units (CWSUs) that are for conditions
just below severe criteria. CWAs are issued for:

Thunderstorms

Turbulence

Icing

Ceiling & Visibility (IFR)

AIRMETs

An AIRMET
advises of weather potentially hazardous to all aircraft but that does not
meet SIGMET criteria.

AIRMETs are issued by the National Weather Service's
Aviation Weather Center (for
the lower 48 states and adjacent coastal waters) for the following weather
impacted reasons:

Instrument Flight Rules (IFR) or Mountain Obscuration -

Ceilings less than 1000 feet and/or visibility less than 3 miles
affecting over 50% of the area at one time.

Extensive mountain obscuration

Turbulence

Moderate Turbulence

Sustained surface winds of greater than 30 knots at the surface

Icing

Moderate icing

Freezing levels

These AIRMET items are considered to be widespread because they must
be affecting or be forecast to affect an area of at least 3000 square miles
at any one time. However, if the total area to be affected during the forecast
period is very large, it could be that only a small portion of this total
area would be affected at any one time.

AIRMETs are routinely issued for 6 hour periods beginning at 0245 UTC.
AIRMETS are also amended as necessary due to changing weather conditions or
issuance cancellation of a SIGMET.